This invention relates generally to the synthesis of voltage and resistance parameters using active circuits and in particular to a voltage and resistance synthesizer capable of synthesizing a wide range of precise voltage and resistance parameters using a digitally controlled pulse width modulation technique.
The manufacturing and process industries employ a variety of electronic instrumentation such as temperature transducers, pressure transducers, and digital panel meters to measure temperature, pressure, flow rates, and other process related parameters. Maintaining such instrumentation and ensuring its accuracy and consistency over time requires calibration. Calibration of instrumentation is typically performed according to regular maintenance and calibration schedules in order to meet government regulatory requirements, to ensure uniformity of industrial processes and manufactured goods, or to ensure conformance with increasingly stringent quality control standards.
Calibrating electronic instrumentation is commonly done by measuring a wide range of known resistance and d.c. (direct current) voltages with the instrumentation and adjusting the instrumentation so that the measured values precisely match the known values. Such known values are obtained from measurement standards that are `traceable` to NIST (National Institute of Standards and Technology). Traceability means that individual measurements are related to national standards maintained by NIST through an unbroken chain of comparisons through a series of intermediate reference standards. Calibration equipment to provide these measurements at the end of this chain are commonly known as calibrators.
Different instruments require different methods of calibration and different values of voltage and resistance. A temperature transducer, for example, converts the millivolt level signal generated by a bimetallic junction of a thermocouple into a corresponding d.c. current level in an industry-standard 4 to 20 milliamp loop. The d.c. current is then measured and converted into a corresponding temperature indication by another instrument. Calibrating a temperature transducer requires presenting a series of known input voltages which duplicate the voltage levels generated by the particular type of thermocouple, making a series of interactive adjustments of the transducer over multiple calibration points, and measuring the output current that is generated by the transducer at each calibration point. Other types of instrumentation may require a simulated resistance for calibration. For example, instruments that convert the temperature-dependent resistance provided by an RTD (resistance temperature detectors) into temperature measurements require simulated RTD resistances for calibration. Therefore, it would be desirable to provide both resistance and voltage parameters that are programmable and synthesized with sufficient accuracy and precision for purposes of instrument calibration.
Furthermore, the calibration of such instrumentation typically requires taking the instrument to be calibrated back to the instrument shop where calibration is performed using high performance calibrators, thereby removing the instrument from service during the calibration period. Separate voltage calibration and resistance calibration standards have traditionally been provided by bench-top calibration equipment whose large size and heavy weight dictate they must remain in the instrument shop. It would therefore be desirable to combine the resistance and voltage standards into one unit, reducing component count and enhancing portability.
An Impedance Synthesizer, using active circuits and employing a programmable multiplying digital-to-analog converter (DAC) for control, was disclosed in pending U.S. patent application Ser. No. 08/160,992, filed Dec. 2, 1993 and assigned to Fluke Corporation, assignee of the present invention. This circuit provides for synthesis of resistance, capacitance, and inductance values based on a single reference resistor, capacitor, or inductor. The resolution of the synthesized impedance value is governed by the number of bits of resolution available in the multiplying DAC employed in the circuit. However, this prior art synthesizer does not provide for the synthesis of d.c. voltage.
Field calibration involves the calibration of instruments "in place" rather than removing them to a central location like an instrument shop for calibration. The ability to take the calibration standard to the instrument to be calibrated would be a desirable attribute in exchange for slightly reduced performance from that of the calibrator in the instrument shop. A Linearity Control Circuit For Digital To Analog Converter is disclosed in U.S. Pat. No. 4,716,398, to Eccleston, et at., provides a high stability, synthesized d.c. voltage using a pulse width modulation (PWM) technique which obtains precision in the synthesized voltage through the precise control of the duty cycle by a digital timing method. A reference voltage is switched on and off using a pair of switches operating at a constant switching frequency but with precise control over the duty cycle. The clocked waveform is fed into a filter network which integrates it over time to obtain an average d.c. voltage. The result is a synthesized voltage that is a precisely controlled fraction of the reference voltage based on the duty cycle. Eccleston et at. teach the use of two additional switches working in tandem with the first two switches of the PWM in order to remove the effects of the switch resistance and improve the linearity of the amplifier thereby improving the accuracy of the voltage generated. For portable applications, however, it would be desirable to provide a voltage and resistance synthesizer which requires fewer components in the pulse width modulator while providing both voltage and resistance parameters of sufficient accuracy for "in place" calibration of different types of instrumentation.